xr-2211

[Scott] and his buddies were having some fun with their handheld transmitters one day when they decided it was time to build some add-on hardware that could transmit and receive location data. They set their sights on a set of Audio Frequency Shift Keying units that could each encoded and decipher location from the counterpart.

The build got off to an easy start, centering around an Arduino board with a GPS module for capturing precise location data. Next it was time to implement AFSK. On the transmitting side this was done by bit banging the output pins. After a look at the resulting signals on an oscilloscope the team was able to tune the firmware for a pretty tight 1200 and 2200 Hz output. But trouble was brewing on the decoding side of the equation.

The first decoding attempt used the FreqMeasure library written by [Paul Stoffregen]. After no success they moved to a hardware solution in the form of the XR-2211 FSK Demodulator chip. It should have been simple, feed it the signals and read the digital output pins to capture the desired data. This is the point at which you need to click the project link at the top to soak in all of the gory details. Long story short, a noisy power rail was causing sporadic performance of this chip. By the time this issue was discovered interest had waned and the project was ditched as a failure. Was there a quick fix that could have salvaged it such as adding a filtering circuit for that chip? Let us know how you would get this back on track by leaving a comment below.

[Thanks Lewin]

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He doesn’t provide a method of tuning the radio signal, but at first you can use the audio samples he points to. The actual broadcasts happen on one of seven frequencies between 162.400 MHz and 162.550 MHz but the tones are also broadcast on TV and Radio alerts. Once you have the audio it is fed into a pair of XR-2211 Tone decoders. This provides just three interface pins for the Arduino to watch.

The annoying noise that grabs your attention at the beginning of a weather alert, or test of the alert system is actually what the SAME data packets sound like. From those tones this system will be able to decode what type of alert is being issued, and the geographic locations it affects. If you interested in more info about SAME head over to the Wikipedia article on the topic.